Behavior of iodate substituted ettringite during aqueous leaching

Abstract

Ettringite [Ca6Al2(SO4)3(OH)12·26H2O] found in cementitious waste forms (CWFs) is known to immobilize iodate (IO3−) via sulfate (SO42-) substitution; however, limited studies have investigated the effects of leaching on the long-term fate and behavior of IO3− retained in ettringites. In this study, ettringite mineral transformation and iodine release from IO3−-incorporated ettringite ((IO3−)-Ett) were investigated using batch experiments to determine the mechanisms likely to drive IO3− release/retention within ettringite in environmentally relevant solutions of varying composition, pH, and alkalinity. (IO3−)-Ett was leached in six simulated solutions relevant to the Hanford Site (Washington State, USA) for a period of 56 days: double deionized water, 24 ppm CO32-, 1400 ppm SO42-, Hanford groundwater, Hanford vadose zone porewater and a cement-equilibrated solution. By monitoring IO3− uptake and removal and characterizing changes in the solid material, incongruent dissolution of ettringite to calcite and oxyanion substitution into the ettringite structure (e.g., SO42- or CO32-) were identified as contributing IO3− release mechanisms. However, the formation of calcium carbonates, e.g., calcite, may re-incorporate IO3− (pH < 10) or prevent the release of IO3− when formed on solution-exposed surfaces of ettringite. These results provide valuable insights into iodine release from CWFs that may be used to improve current CWF formulations for the treatment of iodine-containing nuclear waste.